Portable insulation testing equipment



Sept. 20, 1938. T. R. WATTS ET AL v 2,130,865

PORTABLE INSULATION TESTING EQUIPMENT Filed May 19, 1956 2 Sheets-Sheet l Amplifier 4 WITNESSES: INVENTORS I Jam's H. Fro/res &

Z fh J V I Thomas R. Waffs v BY ' AT NEY Sept. 20, 1938. T. R. WATTS ET AL PORTABLE INSULATION TESTING EQUIPMENT 2 Sheets-Sheet 2 Filed May 19, 1956 w'mnsssss: I #6014 Q7 INVENTORS James H. fizz/res 8 Thomas ff:

, urement heretofore obtainable.

Y are respectively connected in adjacent arms of Patented Sept. 20, 1938 UNITED, STATES PATENT OFFICE ron'ranns msum gg -rns'rmc squir- Thomas R. Watts, Forest Hills, and James H.

Frakes, Wilkinsburg, Pa., assignors to Westinghouse Electric a Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application Maylil, 1936, Serial-No. 80,560

7 Claims. (Cl. 175-183) The present invention relates to testing equipand the other a fixed resistor R4 shunted by a ment for determining the insulation qualities of variable condenser C4, both in series with a rehigh-tension bushings, transformer coils and the actor Lc. A switch II in parallel with L; controls like, and more particularly to a testing equipits eflectiveness. ment which is readily. portable and capable of The bridge balance conductor ll iaconnected giving a reading of the capacitance and powerto an indicating instrument I. through a reversfactor of'a test specimen in the field, as distlning switch II. I guished from removing it from the apparatus The bridge above described is of the so-called with which it is associated and taking it to the Schering type, but is inverted with respect to the laboratory for testing. usual .Schering connections, and includes the in- For some years, equipment for testing the inductance In. The usual Scherlng connections sulation qualities of bushings in the field has been are shown in Patent 1, issued December available but it has been quite complicated in 28, 915. to h p e se d assigned to the character and although capable of being transported by truck to the field it was not as readily portable, or capable of as easy manipulation as would be desirable.

It is an object of the present invention, therefore, to provide a testing set for insulation and dielectrics which is more readily portable than equipment now available but which sacrifices none of the accuracy or practical range of measpany. In the so-called Scherlng bridge the Junetion IQ, of the present invention, would be at ground-potential and the Junction H2 at test potential. These potentials are inverted, as shown in Fig. 1, because the test specimen C1 usually has one side grounded when in service, and because of such inversion it is unnecessary to break such ground connection, that is, the specimen may be tested in its service position. 1

Referring to Figs. 3, 4 and 5, the testing apparatus is housed in a portable metallic casing 22 having a removable lower portion 23, a hinged cover 24 and a switching panel 28.

On the panel 2, the three adjusting pointers 38 are used to vary the resistance of Ra (see Fig.

l) and indicate the efi'ective value of such resistance. The three pointers vary and indicate the eifective capacitance of C4. The indicating instrument I! for indicating the bridge balance is visible through. but not connected with, the panel 26, and its sensitivity is controlled by knob 32 as hereinafter described.

' An aluminum shell 34 having top and bottom covers insulated therefrom, as by strips 35 secured inside the shell, is secured within the casing 22 by insulating brackets ll, the main body of the shell 34 constituting, with the metallic walls of the casing 22, the standard condenser C: of Fig. 1.

The bottom section 23 of the casing 22 contains the transformer I, a suitable fuse and terminal block t to which may be removably connected flexible conductors (not shown) extending upwardly 'through'the casing 22 for connection with an outlet socket 40 on the panel 28 to receive a plug connected to the low-potential supply 4 to energize the low potential winding of the transformer. One side of the high-tension transformer winding is electrically connected to a spring strip 1 secured at one end to the transformer and resiliently engaging an insulated ter- Other objects will appear from the following specification taken in conjunction with the accompanying drawings, wherein:

Figure 1 is a diagrammatic sketch of the circuit arrangement used in the present invention:

Fig. 2 is a schematic showing of the circuit of Fig. 1 in conjunction with other parts of thecomplete testing equipment; Y

Fig. 3 is a perspective view of the encased testing equipment;

Fig. 4 is a plan view of Fig. 3; and

Fig. 5 is a view showing the interior of the testing equipment with the casing shown in vertical section on line V--V of Fig. 4.

Referring to the diagrammatic showing in Fig. 1, the test circuit comprises a step-up transformer 2 the low tension winding of which is energized from a suitable alternating current source 4, such as volts. The highWension winding, for the test potential, which may be of the order of 10 kv., has one side thereof grounded and the other connected to the junction ll of a bridge circuit, the opposite junction I! of which is grounded... The transformer potentials may be varied, of course, to obtaina desired test voltage.

The insulation to be tested, here indicated as a condenser C1, and a standard condenser C:

the structureshown in the bridge between the grounded junction l2 and the junctions l4 and it. Of the remaining two bridge arms, one contains a variable resistor R1.

Westinghouse Electric & Manufacturing Com- 3|, controlled byvknob 32 on the panel-216 (Fig.

minal post'8, mounted in the bottom cover of shell 34, when the casing sections 22 and 23 are secured together.

The electrical connections within the casing 22 are shown moreclearl'y in Fig. 2, wherein elements corresponding to those shown in the other figures have corresponding reference numerals or letters. I

Referring to Fig. 2, the casing 22 is grounded, as at 38, corresponding to the ground at bridge terminal [2 in Fig. 1, and one side of the high-tension winding of transformer 2 is grounded to the casing. The other side of such winding is connected to strip 1- which engages the bottom of terminal 8, secured to but insulated from the bottom of shell 34, when the two parts 22 and 23 of the casing are. together. The junction ill of the bridge is electrically connected to'such terminal 8 within the shell- 34, and the junction I3 is connected to the body, or central portion, of

the shell by a .conductor 39. As above indicated, the body of the shell and the metal casing 22 constitute the fixed condenser C2 of Fig. 1.

A resistor 4|, connected between the botto of shell 34 and the conductor extending to bridge junction 10, is of a value to compensate for the potential drop of the bridge impedance devices.

The bottom of the shell and'the top section are connected by a conductor 45 and constitute guard electrodes at the same potential as the bridge junction l4, by reason of resistor 4|.

The indicating instrument I3, which may be a microammeter, is mounted onthe top cover of the shell 34, as shown in Fig. 5, and is preferably provided with an adjustable shunt resistor 4), to vary its sensitvity. A vacuum tube ampli fier 42 controls the energization of the instrument I9; itsinput being connected to the reversing switch 20 which is controlled by a handle 2| on the switching panel 26.

The amplifier 42 may be of any desired type but -preferably includes a. fllter which will pass only the test frequency. The tube circuits may be energized by a separate battery and controlled by a switch 43 on the instrument panel 28. A

voltmeter 34 for indicating the energization of the tube filaments, maybe mounted adjacent to the microammeter l9 to be visible through an opening in the panel, as shown in Figs. 4 and 5.

All of the bridge equipment, with the exception of the indicating instruments and C1, is mounted within the aluminum shell 34 which eflectively shields it. Fiber rods 48 '(Fig. 5) connect the various handles on the panel with the respective bridge; control elements, and the indicating instruments are mounted on top of the shell 34 out of contact with the panel. The shell 34 being substantially at test potential, this type of construction is desirable for the operator.

The bridge junction I 4 is connected by a con ductor 53, and an insulated test lead 52,- extending through the bottom of shell 34 and through the rear of the casing. section 23 (as viewed in Fig. 3), to the ungrounded terminal of the-test specimen Ci, in this case a bushing-of the'condenser type. The lead 32 is relatively long to permit such connection vithout removing the bushing from its installation.

A conducting shield 54 encases the lead 32, from within the casing section 23 to the bushing. and the inner end of the-shield is electrically connected to the bottom section or guard electrode of shell 34. It is desirable that the shield protection of the cording-1y the resistor 4| is of a value to balance the drop through the bridge circuit to the junction I4.

A guard 33 may encase the shield 34 and lead 52 outside the casing 22. Guard 33 is grounded to the casing 22 and is eflective to eliminate the 'minor electrostatic charge which would ordlnar ily develop on the insulation surrounding the shield 54. Such charge. or the discharge thereof,

has no appreciable effect on the operation oi the equipment, but the guard may be provided to 'avold a. possibly undesirable psychological eilect on the operator.

It is to be understood thatthe shield Maud guard 56 are insulated from'each other and from the lead 52, and the guard. 56 is covered with insulation. The showing of these elements is merely diagrammatic, cables of such construction being readily available.

The connection of the bridge-end oi the lead 32 is necessarily detachable to permit removal of the casing section 23. This is accomplished. as indicated in Fig. 2, by a tubular portion 53 depending from the bottom of shell 34, having an aperture in line with a larger opening in the casing portion 23. The member 33 encloses a relatively stiff conductor 53 exposed through said aperture and connected by conductor 53 to the bridge junction l4. The bridge-end of the test cable is provided with a resilient collar 33 connected to the shield 54, and a flange connected to the end of guard 56. The collar and flange are spaced to engage respectively the member 53 and the casing 23 around the respectiveapertures therein. The test lead52 projects beyond the collar 33 for engagement with the conductor terminal 58. Accordingly, by a thrust movement the test lead with its shield and guard may be properly connected to the test set, and may be detachably secured in such position by a clamping nut 3| which pulls the flange of guard 33 against a threaded nipple surrounding the opening in casing portion 23.

If at any time the bushing or other sample under test breaks down or is short circuited. damage might result to the bridge. In order to prevent this, a discharge device 32 may be provided,

comprising a strip or sheet of dielectric material 33, such as mica, having on one side an electrode connected to the high potential junction ll of the bridge, and electrodes on the opposite side connected respectively to the bridge junctions l4. and I3. In the event of a flashover, the dielectric 33 breaks down establishing a low-resistance path shunting the various impedance devices in the lower legs of the bridge. 03 course other cut-out or protective devices of this general character may be used if desired. 7

In operation, with the test lead 32 connected to the specimen, as in Fig. 2, the amplifier 42 energized and the instrument sensitivity resistor 3| at its lowest point of adjustment, the lead from the low-voltage source may be plugged in the socket 43 (Fig. 4). The transformer 2 is thereby energized, and the high-potential test voltage is impressed on the bushing throughthe bridge circuit. The bridge may then-be balanced-by ad,- justing the resistor R: and condenser 04 to obtain a null reading on microammeter l3, the sensitivity control 3| being operated to maintain the indications of the microammeter within the limits of the scale. By reason of the plurality or. dial -switches 23 and 33 for R: and G4, a desired ne tion may be taken care 6mm- 9 1 and i R. 2 l z Cot 9 is, for all practical purposes, equal to cos 6,

which is the power factor expression, and, in (2), C2 and R4 being constant the capacity of the specimen is readily obtained from the reading of Ra. It is assumed that the series capacitance and parallel capacitance of the specimen are equal for all practical purposes.

The proofs of the above formulae and statements are not given here, in the interest of brevity, but are well understood by those skilled in the art.

When the bridge is balanced and the R3 and C4 readings taken, the bridge reversing switch 20 should be thrown, by means of handle 2| (Fig. 4) to reverse the connections to the amplifier, and the bridge rebalanced, if necessary. If the readings are not the same with the switch 20 in both positions, it indicates the existence of interference afiecting themi'croammeter, such as an alternating current magnetic field. If the readings are not the same, the values of R3 and C; taken in each position of the switch 20 should be averaged to give the true values.

Also, errors due to interference from an electrostatic field, such as from a high voltage line overhead, can be eliminated by averaging the Ba and C4 readings obtained with the low-voltage supply reversed. This may be accomplished merely by rotating the plug in socket 40.

It has been found in practice that electrostatic interference may be of such magnitude that the bridgecannot be balanced. That is, with the C4 dials at, zero, a null reading cannot be obtained on the mlcreter i9. It is an imp rtant aspect of the present invention that this condiof by the impedance In shunted by switch II. {This impedance is so designed'that the value of C4 is increased by such an' amount that it will be sufilcient to compensate ror interference values usually encountered. For example, if it is increased by .3 mid, which is equivalent to 3% power factor, the value will be sumciently larger than errors usually encountered to permit the desired compensation. In reading the power factor from the dials of C4, therefore,

with switch flopen, the following formula obtains:

P. F- C C (with240 reversed) .3 mi LX 0 The R4 values are read as usual.

in accordance with the foregoing, a testing equipment is provided which will read directly power-factors up to 10%. Above this value, it is necessary to revert to a correction chart or curve because the assumption that power factor is equal to the reading of C4 no longer holds with sum cient accuracy. In the great majority of cases.

however, the power factor will be below 10 and the reading may be taken directly from the C4 dials.

Further, a testing equipment constructed, as described, has a weight of the orderof 100 lbs.

and may be readily carried by hand or transported to the field in an ordinary automobile. This renders the use of the testing equipment much more flexible than heretofore possible when trucks had to be used and several men required to move the equipment into position. In spite of the high degree of portability, the accuracy of the test results is not adversely affected and the operation of the set is facilitated by the simplification of the bridge circuit and the, provision of the corrective inductance 1s.

In addition, the mechanical arrangement of the parts results in a particularly compact design. The instruments and dials are readily viewed from the top of the casing, the section I! may be readily removed to afford access to the transformer and the bridge equipment, and the fixed condenser plate 34 extending vertically prevents the accumulation of foreign matter between it and the wall of casing 22 which would tend to introduce losses.

Quite obviously, the construction and circuit arrangement shown may be varied without departing from the scope of the invention, and it is desired that no limitations be placed on the invention, except as imposed by the appended claims.

We claim as our invention:

1. A device for measuring insulation qualities of a grounded specimen comprising a metallic casing, a shell of conducting material therein and spaced therefrom to constitute a condenser of fixed capacity, a source of test voltage and means for impressing it upon said specimen in= eluding a bridge circuit one arm of which includes said specimen and the adjacent arm contains said fixed condenser, adjustable pedance devices mounted in said shell and connected in the other two arms of said bridge and means for operating them to balance the bridge, and for compensating said bridge forerrors resulting from an electrostatic charge on said speci men. I

2. A device for measuring insulation ties of a grounded specimen comprising a metallic casing, a shell of conducting material therein. and spaced therefrom to constitute a condenser of fixed capacity, a source of test voltage and means for impressing it upon said specimen including a bridge circuit one arm of which includes said specimen and the adjacent arm contains said fixed condenser, adjustable impedance devices in the other two arms of said bridge and means for operating them tobalance the bridge, and means for compensating said bridge for errors resulting from an electrostatic charge on said specimen comprising an impedance element of fixed value connected in additive relation to an impedance device in one of the said other two arms.

3. A device for measuring insulation qualities of a'grounded specimen comprising a metallic casing, a sheet of conducting material therein and spaced therefrom to constitute a condenser of fixed capacity, a source of test voltage and means for impressing it upon said specimen including a bridge circuit one arm of which includes said specimen and the adjacent arm contains said fixed condenser, adjustable impedance devices in the other two arms of said bridge and means for operating them to balance the bridge. and means for compensating said bridge for errors resulting from an electrostatic charge on said specimen comprising an impedance element means for impressing it upon said specimen including a bridge circuit one arm of which includes said specimen and the adjacent arm contains said fixed condenser, adjustable resistors and condensers in the other two legs for balancin: said bridge, and'means for over-compensating said balancing means for errors resulting from an electrostatic charge on said specimen.

5. In a device for measuring the power-factor of a grounded insulation specimen comprising an no inverted Bcherlng bridge circuit, means for impressing a high-potential test voltage on one junction thereof, the opposite junction of which is to be grounded, a shielded test cable extending from an adjacent bridge junction, a shell of conducting material surrounding' said bridge and electrically connected to the fourth terminal of said bridge. a grounded casing enclosing said shell constituting therewith a condenser of known value. and means for impressing said test a voltage on the shield of said cable.

6. In a device for measuring the power-factor of a grounded insulation specimen comprising an inverted Schering bridge circuit, means for impressing a high-potential test voltage on one junction thereof, the opposite junction being grounded, a shielded test cable extending from an adiacenfbridge junction, a shell of conducting material surrounding said bridge and elecqualities of .a test specimen comprising abridge circuit, means for impressing a test voltage on one junction thereof, adjustable impedance devices in the arms of the bridge adjacent to said junction. astandard oi comparison in a'third arm of said bridge, means including a shielded cable for connecting a test specimen in the fourth arm of the bridge, the junction between said third and fourth arms being electrically connected to the source of said test voltage, and means for compensating said bridge for errors resulting from an electrostatic charge on the specimen. 1

THOMAS R'WA'IIS. 

